**11. Hydride-halogen systems**

156 Recent Advances in Crystallography

**Figure 16.** Halogen and hydrogen bonds in the structure of [RuI2(H2dcbpy)(CO)2]·I2.[48]

distances, these bifurcated bonds are weaker than the non-bifurcated one.

**Figure 17.** Halogen bonding interactions in [RuI2(H2dcbpy)(CO)2]·2I2.[49]

Ormond-Prout, Smart, and Brammer proposed that halogen bonds can be used to predict and control the process of self-assembly and to fine-tune the electronic properties of cyanometallates.[50] To confirm this assumption they synthesized two types of halopyridium hexacyanometallate salts, (3-XpyMe)3[M(CN)6] and (3, 5-X2pyMe)3[M(CN)6] (X=Cr, Fe, Co).[50] The authors harvested a total of ten crystals, and found out that five out of each family of compounds were isostructural, while other structures were the solvates, (3- IpyMe)3[Fe(CN)6]·2MeCN(2·2MeCN) and (3,5-Br2pyMe)3[Cr(Cr(CN)6]·(10·4H2O). The

Any ligand possessing a free electron pair can be seen as a potential halogen bond acceptor. The N-bound thiocyanate in *cis*-diisothiocyanato-bis(2,2'-bipyridyl-4,4' dicarboxylato)ruthenium(II) provides an example of such a ligand. The structure of the [RuI2(H2dcbpy)(CO)2]·2I2(Fig. 17) adduct have been obtained at room temperature by mixing I2 and the complex in methanol.[49] In this structure, the sulfur atom of one of the thiocyanate ligands forms bifurcated halogen bonds with two I2 molecules. Based on the

> In addition to the more conventional electron donors, the hydride, R-Hδ- , has also been proposed as potential XB acceptors.[51] This type of halogen bonds, R-Hδ-…XRX, has been investigated computationally by analyzing a series of model systems. The results indicate that Hδ is a potential electron donor for halogen bonds.[51–53] The halogen bonding interaction between LiH or HBeH and either XCF3 or XCCH (X= F, Cl, Br, I) has been studied with high-level quantum mechanical calculations, quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) methods. The most important finding of these studies has been that the hydride-halogen bond formation causes the elongation of Rδ+-Hδbond due to the involvement of hydride in the halogen bond. It has been suggested that the interaction is inductive in nature, and the formation of hydride-halogen bond results in the charge transfer from the hydride to a halogen-donor molecule.[51]
